The present invention relates in general to a flow molding process and associated apparatus. More particularly, the invention pertains to the forming of thermoplastic parts using radio frequency heating. The concepts of this invention are particularly adapted to the forming of multiple thickness pieces.
In the construction of the plastic parts using radio frequency heating, it is common to select the plastic material and the diaphragm on the basis of their power factor. Thus, in one prior art example there is employed thermoplastic polyvinylchloride which has a power factor on the order of 0.09. The diaphragm may be of a silicone rubber having a power factor on the order of 0.01. This 9 to 1 difference in power factor means that there will be substantially more heat dissipated in the plastic than in the diaphragm which is desirable. However, when there are different thicknesses of plastic that are being fabricated, the problem is that there will be an uneven distribution of heat in the plastic with the possibility of burnt areas especially in thicker sections of the plastic.
Accordingly, it is one object of the present invention to provide an improved method of molding plastic pieces and in particular those having non-uniform thickness throughout wherein a uniform heat is applied to the plastic piece throughout.
Another object of the present invention is to provide a process for forming plastic pieces of non-uniform thickness without causing a burning of the piece in particular in thicker areas thereof. It is also not uncommon to have a burning in a thicker sections while the thermoplastic in a thinner section is not sufficiently heated to flow properly.